Houston residents being evacuated Tuesday by volunteers from San Antonio.

Tuesday afternoon, with floodwaters rising in Houston and residents still stranded in their homes, hundreds of people turned to the walkie-talkie app Zello, which posts short voice messages in an ever growing feed, to coordinate help.

One Zello group, Houston Harvey, posted hundreds of messages an hour. Among boat owners offering their services and drivers in need of a safe route out of town, much of the chatter focused on Heidi, a mother with an unspecified illness and two autistic sons, one of whom uses a wheelchair.

“She’s on the emergency evacuation list several times over,” said a woman with the username mswrt. “We’re trying to get somebody out to her as fast as we can.”

“What is the location for that mom with the autistic kids?” asked a man who used the handle yert68. “We have a low-water boat able to get to her, and we’re in the Baytown area right now.”

A third user, sarah1118, gave the address. “She is in an apartment complex that is slowly rising. And she is alone with her two sons and she needs a boat that can accommodate her son’s wheelchair.”

But Zello’s limitations were quickly apparent. It didn't offer a way to respond directly to other users if they hadn't been added as a contact. New messages, posted in real time, overwhelmed older ones. Some gave advice that only might be helpful. Others were just dead air. A quiet man who offered no other information simply asked “How can I help?” No one responded to him. Clearly, no one was in charge.

“The Coast Guard is in that area,” said a user identified as CW2009. “I’m not sure if they’re rescuing them.”

Eventually, according to a woman who said she was Heidi’s sister, the Coast Guard came for Heidi, but they didn’t rescue her. Misty, who said she was in touch with the Coast Guard, said it was because they couldn’t take the wheelchair, and Heidi wouldn’t, or maybe couldn’t, take her son out of it.

“It’s ridiculous. You have two kids and an adult that needs medication. No food, needs supplies, and they left her there behind,” the sister said.

Another user weighed in: Did the Coast Guard help?

The response: “They’ve already left the area.”

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That Texans have turned to social media in the wake of a historic flood shouldn't be surprising. As of Monday, according to the Federal Communications Commission, 16 of the area's emergency call centers were having problems dealing with the deluge of 911 calls they were receiving. That sent people needing help to Twitter, Facebook, and Zello to summon help.

But while that's potentially valuable, it is an imperfect way for governments to help disaster victims, Michael Lindell, a professor emeritus at Texas A&M University and the former director of its Hazard Reduction and Recovery Center, told BuzzFeed News.

For one, he said, “You’re not going to wind up with 100% of the local population reading the local emergency management’s Facebook pages.”

For another, social media, with its thousands of users, offers little consistency in standards and practices, making dependence on it risky. Apps such as Zello, better known as a protest organizing tool in countries such as Turkey, Russia, and Venezuela, aren't well known in Texas, and the app itself is a major drain on a smartphone battery.

In disaster areas, the internet can be as unreliable as any utility. Nearly 200,000 people have lost internet in their homes due to Harvey, according to the FCC, and internet service on cell phones has been hampered, with 364 cell phone towers in 27 counties in Texas and Louisiana suffering at least partial service outages.

NetBlocks, a research program that grew as a response to censorship in Turkey and now monitors internet outages around the world, noted that some areas, like Corpus Christi, suffered severe internet outages when the storm hit.

Others, like the city of Victoria, southwest of Houston, have suffered sustained trouble getting online. Dyn Research, an Oracle property that studies connectivity, found correlations between power outages — widespread in Harvey’s wake — and loss of internet connection.

That hasn’t stopped Texas authorities from trying to use social media to coordinate a hodgepodge attempt to connect aid workers and those in need of help. But even as they used it, they recognized its limitations.

At one point, Ed Gonzalez, the sheriff in Harris County, which includes Houston, requested that citizens in need of rescue stop tweeting him and call 911 instead, even though he admitted that it was at times impossible to get through.

Still, he used social media himself. On Sunday, he tweeted that a pregnant woman on Angelo Street was going into labor and needed help, and tagged the Houston Fire Department. Later, he tweeted that she’d gotten an ambulance.

But Twitter wasn’t a great system: Another woman tweeted to him, adding a screengrab of a Facebook comment, that another pregnant woman needed help. Gonzalez tweeted her address, tagging the Houston Fire Department and City of Houston Office of Emergency Management. But it wasn’t clear if they saw the tweet and were able to help, and he didn’t mention her again.

Conflicting and inadequate communications echo 2005’s Hurricane Katrina and its aftermath, which killed 1,833 people, displaced 600,000, and cost an estimated $130 billion. A congressional report on Hurricane Katrina found that inoperable or damaged communications systems drastically exacerbated problems caused by the storm.

New Orleans police didn’t have functional communications for three days, and for a period, first responders were restricted to using only two radio channels on a backup system.

Louisiana state police found that damaged towers for its radio system, used by 70 agencies and 10,000 users and last updated in 1996, “severely hampered the ability of emergency responders operating on the state system to communicate with other emergency services personnel.”

That, of course, was before the creation of Twitter in 2006 or the introduction of the iPhone in 2007. The Houston flooding from Harvey makes clear both developments have changed the way disaster communications can work.

At the Digital Operations Center of the Red Cross in Dallas, volunteers monitored social media distress calls using in-house software that pulled data from Facebook, Twitter, and other websites using search terms like “hurricane” or “storm.”

The in-house tool clustered the social media data it pulled in various ways — word clouds, heat maps, most tweeted posts, and most popular images and videos — and allowed Red Cross workers to monitor the needs of people affected by disasters like Harvey closely.

“Social media is extremely important to the Red Cross because it allows us to connect to more people,” said Krysta Smith, a digital communications specialist. “It's a live feed that lets us know immediately what we’re facing, what residents are facing, and what actions need to be taken.”

But it has its limitations. The geolocation function can be inaccurate, for one.

Still, it shows the ways first responders could make better use of the last decade's advance in personal communications – if public officials and taxpayers are willing to commit to making it happen.

“Society has priorities,” Lindell said. “Could you develop evacuation plans for everybody in Houston? Of course. But the question is how likely is that to happen? How much are you willing to pay now for the capacity for something that might not happen for another 150, 250 years?”

Alex Kantrowitz contributed.

Correction: The internet outage research program is called NetBlocks, not Global Blocks.